In an apparatus for controlling digital data (hereinafter referred to as a host device) such as a digital camera, movie, portable music player, and the like, as a storage device for retaining digital data, there is a memory card mounting a nonvolatile memory. As a nonvolatile memory used by the memory card, a flash memory is commonly used. Since the storage device is kept in a busy state, seen from the host apparatus, until completion of writing to the flash memory because the flash memory takes long time for writing, the host device cannot be free from the storage device. In addition, if the power is shut down during the writing, a state of a memory cell in being written is not assured. In Patent document 1, a method, including a faster nonvolatile memory compared to the flash memory, for writing data to the faster nonvolatile memory when the power failure is detected during the writing to the flash memory.
Patent document 1: Japanese Unexamined Patent Publication No. Hei 11-143788.